Polymerization of conjugated dienes, especially butadiene-1,3, is well known in the art using a variety of catalysts and a variety of solvents. Thus U.S. Pat. No. 3,135,725 discloses cobalt salt-hydrocarbyl aluminum catalysts and U.S. Pat. No. 3,046,265 discloses alkyl aluminum, cobalt halide and acetyl halide catalysts, both employing a variety of diluents including aliphatic, cycloaliphatic and aromatic hydrocarbons. U.S. Pat. Nos. 3,094,514 and 3,646,001 even suggest the use of a mixture of aliphatic and aromatic hydrocarbons but clearly stating that the mixture must contain at least 15% by weight of an aromatic solvent. The former patent notes that the presence of the aromatic hydrocarbon produces somewhat faster reactions.
Although generally the order of addition of the reactants and the catalysts apparently was not considered to be of critical importance, various patents disclose different orders of addition. Thus in British Pat. No. 926,036 the reactants and catalysts were added in the order of a solvent, an aluminum compound, cobalt chloride in pyridine and finally butadiene while in British Pat. No. 924,427 the order was: an aromatic solvent, cobalt and then aluminum compounds as catalysts, an additional aromatic solvent, an aliphatic solvent and finally butadiene. U.S. Pat. No. 3,284,431 discloses adding to the reaction vessel an aromatic solvent, butadiene, an aluminum compound catalyst, an activator and finally a cobalt compound catalyst. Another order of addition is disclosed in U.S. Pat. No. 3,646,001 where cobalt and aluminum catalysts, such as cobalt octoate and diethyl aluminum chloride, are prereacted in wet benzene at a temperature below 20.degree. C. and then are added butadiene and additional solvents such as benzene and butene-1. Published Japanese patent application SHO-44-10276 discloses dissolving a cobalt catalyst in dry toluene, feeding butadiene-1,3 to the reaction vessel and then adding an aluminum catalyst to begin polymerization.
As a rule the most preferred solvent for butadiene polymerization has been benzene. However, from the point of view of health safety, benzene is an undesirable material as evidenced by recent severe restrictions placed by the Occupational Safety and Health Agency (OSHA) on the concentration of benzene permissible in the work area. For this reason it is imperative to find a solvent or solvent combinations for butadiene polymerizations that would yield cis-1,4-polybutadiene having the same or better properties than those of polybutadiene prepared in benzene.
The difficulty with most solvents is, however, that the polymerization rate is too fast making the reaction practically uncontrollable, or it is too slow and therefore not economically feasible. The initial rate of conversion (polymerization) of the monomer is of critical importance since a difference of only a few percentage points may make the difference between having a controllable and an uncontrollable polymerization. Furthermore, if the polymer is formed at a certain polymerization rate it will not have the required desirable properties.